disc-shaped structures such as the chloroplast. The slow axis of trans- 

 mission of a collagen fiber is found to be parallel to the long axis of the 

 fiber and, hence, is regarded as showing positive birefrigence with respect 

 to its long axis. Likewise, the fibers of the mitotic spindle exhibit positive 

 birefrigence with respect to their long axes (see Chapter 6, Figure 6-2). 

 In the case of the chromosome, however, its slow axis of transmission 

 is perpendicular rather than parallel to its long axis, and is said to show 

 negative birefrigence with respect to its length. 



The birefrigence of biological materials may be due to either (1) 

 a regularity in pattern of molecular structure which produces intrinsic 

 birefrigence, or (2) a preferred orientation of asymmetrical particles 

 which produces form birefrigence. Materials which exhibit birefrigence 

 when subjected to mechanical stress are spoken of as displaying strain 

 birefrigence. For example, the coiled macromolecules of rubber become 

 uncoiled and oriented when put under stress to produce the strain bire- 

 frigence exhibited by this material in the stretched condition. Many 

 crystals show intrinsic birefrigence, which is related directly to the par- 

 ticular arrangement of atoms in the crystal lattice. Form birefrigence 

 contributes appreciably to the total birefrigence shown by many cell 

 structures. Two possible arrangements (Figure 11-26, A and B) which 

 may occur in biological materials and produce form birefrigence are (A) 

 elongated particles oriented parallel to a long axis (fiber axis), and (B) 

 flat, thin discs which are stacked one upon the other as grana are stacked 

 in the plant chloroplast. To determine whether a particular birefrigent 

 object exhibits intrinsic or form birefrigence one simply immerses it in 

 solutions of different refractive index. If the object shows form birefrig- 

 ence, a solution of specific refractive index will be found which matches 

 that of the asymmetric particles and extinguishes the birefrigence of the 

 object. Intrinsic birefrigence, on the other hand, is unaffected by varia- 

 tions in the refractive index of the surrounding medium. This method 

 permits one to estimate the degree to which both intrinsic and form bire- 

 frigence contribute to the total birefrigence of a particular cell structure. 



Polarization microscopy is a very sensitive tool for the study of molec- 

 ular orientation, but is limited by the fact that it tells the observer little 

 about the properties of molecules other than their orientation. Most of 

 the polarization studies dealing with biologic material have been di- 

 rected toward study of the structure of the mitotic spindle. This type of 

 approach has probably given the most complete information concerning 

 the structural organization of the spindle and the changes it undergoes 

 during the various stages of the mitotic process. 



SURVEY OF CYTOLOGICAL TECHNIQUES / 247 



